1. Field of the Invention
The present invention relates to a photochromic composition which can be polymerized and cured upon irradiation with active energy rays such as ultraviolet rays to exhibit favorable photochromic property.
2. Description of the Prior Art
Photochromism is a phenomenon which has drawn considerable attention in the past several years and is a reversible action exhibited by certain compounds. That is, a compound quickly changes its color when it is irradiated with sun light or light containing ultraviolet rays such as of a mercury lamp and returns to its original color when it is no longer irradiated with the light and is placed in a dark place. A compound having this property is called a photochromic compound, and compounds having various structures have heretofore been known without, however, any common skeleton among their structures.
A polymer that exhibits photochromic property is obtained by a method of applying a photochromic compound onto the surfaces of a polymer that has been formed in advance and by a method in which the photochromic compound is dissolved in a radical polymerizable monomer which is then polymerized.
The radical polymerizable monomer can be generally polymerized by either a method in which the polymerization is accomplished with heat or a method in which the polymerization is accomplished with light. When the radical polymerizable monomer dissolving the photochromic compound is polymerized with light to obtain a polymer having photochromic property, however, the photochromic compound contained therein absorbs ultraviolet rays which are necessary for polymerizing the radical polymerizable monomer, and the transmission of light is hindered by color developed by the photochromic compound itself upon irradiation with the light, making it difficult to favorably polymerize the radical polymerizable monomer. According to the present inventors, furthermore, it has been known that when the photochromic compound is dispersed in the insufficiently polymerized matrix polymer as a result of employing the photo polymerization, the obtained photochromic property lasts for only very decreased periods of time (life is shortened).
Because of these reasons, therefore, the photochromic polymer is obtained, usually, by polymerizing the radical polymerizable monomer in which the photochromic compound is contained (dissolved) with heat. According to this polymerization method with heat, however, the polymerization time usually lasts for several hours, which is not satisfactory from the standpoint of productivity.